As an installation (such as a greenhouse) constituted by a transparent roof material or a transparent side wall material, wherein plants are cultivated, there is an installation, of which the roof or the side wall is constituted by a transparent synthetic resin film. In the present invention, such an installation for cultivation of plants is referred to as an agricultural house, and a transparent synthetic resin film to be used for the roof or side wall is referred to as an agricultural film.
With an agricultural film spread to form an agricultural house, if dew condensation or fogging takes place on its surface during the cultivation of plants, the transmittance of sunlight may be deteriorated, and the growth of plants may be adversely affected. Accordingly, it has been attempted to impart hydrophilicity to the surface of an agricultural film for the purpose of improving the anti-fogging property, wettability (property whereby water droplets are uniformly wettingly spread to form a water film), anti-fouling property, etc.
The following ones are known as agricultural films having hydrophilicity imparted to the surface.
(1) One obtained by applying an anti-fogging composition comprising silica sol and alumina sol having different average particle diameters, a surfactant and a liquid dispersion medium on a surface of a base material film, followed by drying to form a coating film (Examples 4 to 6 in Patent Document 1).
(2) One obtained by applying a coating fluid comprising colloidal alumina, colloidal silica and an anionic surfactant in a specific ratio, on a surface of a base material film, followed by drying to form a coating film (Examples 1 to 7 in Patent Document 2).
(3) One obtained by applying a coating anti-fogging agent comprising an aqueous medium, colloidal alumina, colloidal silica and a water-soluble resin in a specific ratio, on a surface of a base material film, followed by drying to form a coating film (Examples 1 to 14 in Patent Document 3).
(4) One obtained by applying an inorganic coating composition comprising alumina particles, of which an average of aspect ratios of agglomerated particles in a dispersion medium is from 3 to 20, an average particle diameter in a long axis direction is from 100 to 500 nm, and an average particle diameter in a short axis direction is from 2 to 100 nm, silica/alumina composite particles, of which an average particle diameter of agglomerated particles in a dispersion medium is at most 150 nm, and water, on a surface of a base material film, followed by drying to form a coating film (Patent Document 4).
However, in the agricultural films (1) and (2), alumina particles and silica particles are merely attached by a surfactant on the surface of a base material film, and the adhesion between the coating film and the base material film is inadequate. Accordingly, the abrasion resistance of the coating film is inadequate. Further, the water resistance of the coating film is low, whereby the hydrophilicity cannot be maintained for a long period of time.
In the agricultural film (3), the coating film contains a water-soluble resin (binder), whereby the adhesion is improved as compared with the agricultural films (1) and (2), but the adhesion between the coating film and the base material film is still inadequate. Accordingly, the abrasion resistance of the coating film is inadequate. Further, the water resistance of the coating film is low, and the hydrophilicity cannot be maintained for a long period of time.
In the agricultural film (4), as compared with the agricultural films (1) to (3), the adhesion between the coating film and the base material film is high, and the abrasion resistance of the coating film is good. Further, in an accelerated weather resistance test, the hydrophilicity can be maintained for a long period of time. However, in a real outdoor exposure test, the hydrophilicity abruptly drops at some time. Therefore, it is desired to further improve the durability of hydrophilicity (hereinafter referred to as hydrophilic durability).